The mechanical properties of materials can be examined by means of several principally different methods that are used in engineering design as well as quality control and life estimation of machine parts and structures. The methodological approaches can be divided into destructive and nondestructive methods, where the latter is an alternative method since it allows for the execution of tests directly on a structural component and/or even during operation without the need of manufacturing the special standardized samples. Nondestructive methods are usually less time-consuming, less labor-intensive, and they are energy-efficient. This is possible due to high sensitivity, resolution, and stability of the sensing elements of the testing device, coupled with its compact design and advanced automation on the hardware and software levels. A relatively simple non-destructive test based on the instrumented indentation method provides the user with a wide range of material properties such as hardness, modulus of elasticity, yield strength, tensile strength, and fracture strength, energies of elastic and plastic deformations, strain hardening exponent, etc.
The hardness measurement is normally carried out by the indentation method, in which an indenter with defined geometric parameters penetrates into the surface of the test sample. The hardness number is determined by measuring the geometrical parameters of the indentation made by the indenter. For example, indentation diameter, indentation depth or displacement of the indenter at the given load, are among the measured values. Other mechanical properties of the material can be subsequently obtained from the measured numerical values of hardness using known and standardized relationships.
In the instrumented measuring systems based on the indentation method, the accuracy of measured penetration depth (linear displacement) of the indenter is important. This accuracy depends on the sensitivity of the device, location of the displacement sensor and its resolution. Relatively small displacements are usually measured by the use of lasers, strain gauges, electromagnetic sensors etc. The measurement accuracy and reproducibility of the results can be influenced by a number of factors, including the location of the displacement sensor as well as the distance and the number of other elements embedded between the sensor and the indenter, which stress-strain states may depend on the loading conditions and the number of cycles, etc. The loading conditions during the test should not affect the functioning of the sensors and indentation device; therefore, a proper design of the indentation device is needed to eliminate the excessive loads of its parts.
Among the disadvantages of known instrumented indentation devices are their use in the limited loading ranges, and/or the types of available indenters are also limited.
Patent CZ 304637 describes an indentation device that comprises a housing in which there is an adapter with a spring, a displacement sensor, and a load cell. The displacement sensor is equipped with a deformable strip and a strain gauge, cylinder, and a rod extending from the indenter to the displacement sensor. The load cell is equipped with multiple deformable strips and the strain gauges. The advantage of the displacement sensor is in its shape, which is formed as an exchangeable capsule that has several possible designs, including a lever multiplier, a deformation bridge or deformation strip, all provided with the strain gauges and an attachment place for the rod. The load cell is also exchangeable and has deformable strips with the strain gauges designed to measure the low loads, and a cylinder with the strain gauges designed to measure the high loads, all connected to an analog signal amplifier and a computer. The adapter can be used to connect the measuring head to various loading mechanisms such as a hardness tester, tensile machine, lever mechanism, etc. The measurement system is also equipped with an analog-digital converter and a computer with a software module, which is used to record the measured parameters of the indentation and for calculation of mechanical and physical properties. The disadvantages of this indentation device is in its large dimensions, it requires the use of mechanical loading devices, which implies that its manual application during measuring is not possible, and that the accuracy and reproducibility of the measurement is not within a sufficiently wide measuring range.
The possibility of counting the mechanical properties such as modulus of elasticity, yield strength, etc. is mentioned only generally, with the measured data of indenter penetration depth and applied force, without the description of a specific method.
Document U.S. Pat. No. 6,718,820 describes several measurement systems, including portable systems that can be attached to the tested body, or to a system with a movable table, or to an assembly with its own loading mechanism, in which the indenter displacement sensor is located next to the indenter and their geometric axes which are parallel to each other. This design may generate an error during the measurement of the indentation depth, especially at the loads as high as 3000 kgf, due to deformation of the components adjacent to the indentation device. This may be the indenter, the indenter holder, etc., thereby reducing the accuracy of the measurement. The maximum load of the load cell is 300 kgf with the deviation of 2.5 to 5.6 g. Such a low load forces limit the use of larger indenters, e.g. with a diameter of 10 mm and larger.
Patent application US 2011/0174036 describes a measuring device for curved samples, where between the table and the sample there is inserted a sphere with the appropriate diameter so that during the indentation the sample positioned correctly and the correct values are measured. There are three displacement sensors placed around the indenter at the angle of 120° to increase the accuracy of measurement of the displacement, where the final signal is averaged. This design does not address the issue of the deformation of the components surrounding the indenter. The use of three sensors significantly increases the cost, complicates the design, and increases demands on handling.
Patent application US 2012/0304750 describes a portable measuring system that fits in the palm and the load can be induced by the hand, or robot, or by a manipulator. The displacement sensors are located in the same manner as in US 2011/0174036.
The device according to U.S. Pat. No. 4,852,397, among similar features of the other designs, also has an ultrasonic sensor for measuring the size of the cracks in the vicinity of the indentation and also describes its own methodology for calculating the residual stresses. The displacement sensor is similar to those used in other systems, and is located next to the indenter, wherein the ultrasonic sensor, indenter, and displacement sensor are in one line. This greatly increases the minimum size of the measurable sample, since these elements must touch the sample surface. Another disadvantage is that the sensor may be damaged by improper handling of the sample. The patent gives its own methodology for calculating the mechanical properties of materials and plot of the stress-strain diagram.
U.S. Pat. No. 6,134,954 describes an indentation device which consists of two parts. In the top part there is a load cell, an indenter, and a displacement sensor. In the bottom part there is a sample holder and a mirror for the displacement sensor. The bottom part is placed on a movable table to move the indenter to the specimen. Parts of the indentation device can be bolted to the loading mechanism. The displacement sensor is located out of the load axis and its distance to the indenter is large, this design is also limited to the loading range of up to 500N with an accuracy of 0.02 to 0.1 N, and the accuracy of the depth measurement is in the range of 0.1-0.5 microns. The patent describes its own methodology for calculating the mechanical properties of the material and the plot of the stress-strain diagram.
The indentation device ZWICK (available from www.zwick.cz/cs/media.html), based on patent GB 2161279 A and other patent documents, has an indentation device with integrated loading mechanism, housing that comprises a displacement sensor with a resolution of 0.02 microns. The displacement of the indenter is measured with respect to the housing using a scale located on the indentation surface. Measurement of the indentation geometry is provided by an optical system in combination with a manual or motorized table. The elastic deformations during the test may negatively influence the accuracy of measurement. The accuracy of the load cell is 1% with two measuring ranges, 2 to 200N and 5 to 2500N. The device enables the use of different indenters and is fully automated, but besides the hardness and plot of indentation curve, there are no other mechanical parameters calculated. The device is large, with a height of about 1 m and weighing about 100 kg.
Document U.S. Pat. No. 4,435,976 describes a device for measuring the hardness of a material with an indentation device which is connected to the operating member for the application of the first preset lower load force followed by the subsequent application of a second preset greater load force. The central rod of the indenter displacement sensor is divided and the strain gauges are placed in the axis of the rod. The rod is mounted in bearings permitting its movement in an axial direction while from the top, hydraulic pistons push on it with different preset load forces. The disadvantage of this design consists in the necessity of using a special hydraulic circuit for applying the load force and in that the rod of the displacement sensor is affected by the load force.
Document WO2013135026A1 describes a portable digital device for measuring the hardness of a material and also has a support saddle, a compression head, an electronic circuit, a digital display, and a device for measuring pressure and the depth of indentation. This device consists of a rotary hand wheel, rotary encoder, sleeves, and a micrometer composed of a nut and bolt. The rotary encoder is mounted on the support saddle. The rotating shaft of the rotary encoder is coupled to the micrometer bolt and rotates with it. The upper end of the micrometer bolt is connected to a rotating wheel, the bottom end is connected to a pressure gauge, and the lower part of the pressure gauge is connected to the pressure head. The rotary wheel, micrometer bolt, device for measuring pressure, and the pressure head are all interconnected and have a common longitudinal axis, and the movement is provided by the rotation of the rotary hand wheel in the axial direction. The advantage of this device is in its simple design and manipulation. The device allows for the measurement of only hardness values, and with low accuracy.
The disadvantages of known indentation devices and instrumented measuring systems used for measuring mechanical properties of materials by the indentation method consist mainly in the fact that there is no device available that would completely eliminate the influence of deformation on the measurement of displacement caused by the action of the load force, which would also permit the testing of product materials using a nondestructive method in a wide range of load forces, e.g. from 1N to 35 kN with adequate accuracy and reproducibility of results, which would be small in size and low-weight while allowing connection to the various loading mechanisms including manual application, and which would be compact with an effective protection of all sensors so that the device would be usable even in aggressive environments and would withstand rough handling.